What time map should I give?
You don’t need to use Time Remapping at all if all you care about is hitting the fps target you want in your final render. Even if your computer isn’t powerful enough to render the animation in real-time, that doesn’t affect the speed of the final render at all: it will take longer to render the animation, but when it’s played back, it will be at the appropriate speed.
Time remapping is used to artificially lengthen or shorten the real time duration of an animation, which could allow you to, for example, render an animation in slow motion. But it doesn’t affect the fps of the rendered animation at all; it just makes the animation happen over more or fewer frames.
If all you want is a 24 fps animation, don’t bother with the Time Remapping settings, just select “24 fps” as your scene’s frame rate and hit render.

If your problem is that when you render your animation actually runs too quickly (in real time) because you timed everything with the 12 fps you were getting in the viewport in mind, then you can use Time Remapping to help. The Time Remapping options just specify a scaling ratio on the duration, where the duration of the final render is effectively multiplied by $\frac{new}{old}$.
Therefore, if you want the final render to run at half the speed of what your timeline specifies, you can put any value in for Old and twice that value in for New. For example, either of these configurations would work, since they both do the same thing:


Note that you’ll also have to double the value you set for your End frame, or your rendered animation will end halfway through.